1. Field of the Invention
This invention pertains generally to plasma processing and more particularly to a system capable of producing a spatially uniform sheet plasma over a large area.
2. Description of the Prior Art
There are many processes which utilize chemical and physical surface modifications activated by plasma. These methods include etching to remove surface material, utilizing plasma-enhanced chemical vapor deposition (PECVD) to deposit new material on a surface, modifying surfaces chemically by anodizing or nitriding, physically altering by ion implantation, or heating a surface by annealing via radiation or particle bombardment. In some cases, such as semiconductor etching and some types of PECVD, it is essential that the plasma be close to the surface so that energetic ions can be drawn from the plasma toward the surface thereby activating chemical reactions. In these cases the substrate can be biased using direct current (dc) or radio frequency (rf) electric fields on the substrate or backing plate to control the ion bombardment energy. In other cases such as diamond deposition and photoresist ashing, the surface is kept out of the plasma so that it is subject to chemical action of neutral radicals generated in the plasma, but is not bombarded by ions.
There are essentially four different general categories of non-thermal plasma sources, i.e., T.sub.e &gt;&gt;T.sub.i, T.sub.n, presently in use or under industrial development for plasma processing. These are capacitively coupled, electron cyclotron resonance (ECR), helicon, and inductively coupled (IC) sources. The type of source used most widely for industrial processing is the capacitively coupled rf reactor. It consists of an electrode, upon which the surface to be processed can be mounted, connected to an rf source. The entire processing area is contained in a low pressure chamber complete with process gas feeds and a gas pumping system. The rf energy is capacitively coupled into a plasma which fills the entire chamber. In some forms of capacitively coupled devices, known as magnetically enhanced or magnetron sources, a magnetic field is used to partially confine and enhance the plasma in a volume adjacent to the electrode surface. Capacitively coupled rf reactors can produce a uniform plasma over hundreds of square centimeters area with ion density .ltoreq.10.sup.10 cm.sup.-3. The same rf which produces the plasma in this type of reactor also generates a bias voltage between the plasma and the surface. This bias is the result of the different mobilities of the ions and electrons that form the plasma. This bias accelerates plasma ions toward the surface. The ion bombardment energy and the plasma density both increase with rf power injected into the reactor. Adjustments to the density can be made by changing the neutral gas density and rf power level but adjustments are limited due to the fact that the single rf source produces both the plasma and the bias voltage on the electrode.
For modern large area applications, higher ion density and lower ion bombardment energy than can be produced by capacitively coupled rf reactors are needed for some applications. Such higher density plasmas can be produced by ECR, helicon, and IC plasma sources. These plasma generators decouple the plasma production to some degree from the processing and allow one to independently control the ion bombardment energy using rf or dc bias on the surface to be processed. These sources suffer, however, from size and uniformity limitations. The sources can also be somewhat inefficient in that they heat the entire electron population to produce and maintain the desired plasma density near the surface to be processed. They also generate large volumes of plasma outside the processing region which shed energy to the surrounding surfaces.
In all of these devices the details of the plasma distribution are influenced by the energy source, the geometry, the neutral gas density, etc. The available plasma distribution depends on a large number of parameters, all of which may have to be tuned to produce a desired plasma condition. Under many circumstances compromises must be made between different parameters restricting the operating conditions available for processing.
Uniformity of feedstock gas and efficient removal of reaction products are also issues that limit the useful area in existing processing systems. Increasing processing area is extremely important to maximize throughput, and also to permit processing of large objects such as flat panel displays.
Another characteristic of existing processing systems is steady state operation. This is a natural mode of operation in typical cases where it takes minutes or longer to perform a particular processing operation. Recently, however, there have been indications that important features, such as the chemical makeup of the plasma or the deposition of charge on the substrate, can be controlled by pulsing the plasma density or bias voltage. Most of the existing sources cannot easily be pulsed on and off on the required fast (0.01-1.0 ms) time scale.